Journal of The Electrochemical Society, 153 1 A1-A4 20060013-4651/2005/153 1 /A1/4/$20.00 The Electrochemical Society, Inc.
Effects of Bulk Photoconductivity on Photocurrent Action Spectra of Molecular p-n Heterojunction Solar CellsOleg Shevaleevskiy,a,b,z Liudmila Larina,b Seung Yeop Myong,a and Koeng Su Limaa b
Department of Electrical Engineering and Computer Science, Korea Advanced Institute of Science and Technology, Daejeon 305-701, Korea Institute of Biochemical Physics RAS, 119991 Moscow, Russia
We have investigated how bulk photoconductivity inuences the photovoltaic parameters of zincphthalocyanine-fullerene ZnPc/C60 p-n heterojunction solar cells. The results indicate that the photocurrent action spectrum of a cell depends strongly on the photoconductivity and spectral characteristics of each component material. We therefore propose a model that simulates the action spectra of the short-circuit photocurrent in the molecular organic solar cells, and our model is based on the assumption that the photocurrent action spectrum depends on the bulk layer photoconductivity. 2005 The Electrochemical Society. DOI: 10.1149/1.2126576 All rights reserved. Manuscript submitted February 26, 2005; revised manuscript received June 30, 2005. Available electronically November 2, 2005.
Molecular organic photovoltaic devices based on p-type semiconductors, such as metal-phthalocyanines MPc where M = Zn, Cu, TiO, or H2 , have recently attracted great interest due to the appearance of n-type fullerenes C60 , which form photosensitive p-n junctions with MPc.1-4 The efciency of organic solar cells is still lower compared to traditional solid-state devices.5 Initially, a relatively large power conversion efciency for a p-n junction organic photovoltaic cell based on p-type CuPc and n-type perylene derivative was realized by Tang.6 Fullerenes were successfully applied in different constructions of molecular photovoltaic cells for fabricating p-n heterojunctions,1,2 improvement of organic cell performance by doping,7,8 and for manufacturing plastic-type solar cells.9 Recently, a record efciency of around 4% was reported for p-i-n multilayer devices that used ZnPc and C60 modied molecular layers.10 However, the power conversion efciency of any molecular photovoltaic device is limited by poor charge carrier mobility 1 cm2 /V s and small exciton diffusion length LD 10 nm in organic layers.11,12 In condensed phthalocyanine layers, exciton diffusion length may depend on the fabrication conditions, and the reported LD values vary from 10 to 30 nm for ZnPc7,13 and from 12 to 60 nm for CuPc lms.14,15 The photoexcitations created in the bulk of a thick molecular layer do not reach the interface region of the p-n junction and therefore do not contribute to the photocurrent of the device. Thus, the carrier production regions are limited to the dimension of space-charge depletion layers that only extend over the lm interfaces. Due to this the light illumination produces two kinds of photoeffects: photoconductivity in the bulk of the layer that decreases the cell serious resistance, and a photovoltaic effect in the interface region resulting in the generation of the photocurrent. Since Simon and Andre16 reported signicant information about photogenerated carriers at the p-n heterojunction, various models have been proposed that simulated the photocurrent of organic molecular solar cells.17-20 However, the role of bulk layer photoconductivity is still being debated. We assume that the spectral dependence of series resistance in molecular organic cells should not be neglected because of its inuence on the cell photocurrent. Normally, the thickness of a bulk layer in organic solar cells several times exceeds the dimension of a depletion layer. Hence, the series resistance can severely limit the shape of photocurrent action spectrum and the conversion efciency. To prove this supposition, we report on the performance of fabricated n-C60 /p-ZnPc solar cells. We also propose a theoretical model that describes the behavior of photocurrent action spectra and effects of the bulk layer photoconductivity.
Experimental Thin molecular layers were vacuum sublimated at a pressure of 106 Torr from resistively heated quartz crucibles using conventional ZnPc and C60 99.9% purity powder purchased from Kodak and MER Corp., respectively. Before the deposition, ZnPc powder was predominantly recrystallized under argon ambient by train sublimation. Figure 1 shows the arrangement of the fabricated p-n heterojunction solar cell produced by successive evaporation of C60 and ZnPc. First, we deposited n-C60 layers on Corning glass substrates coated with indium tin oxide ITO transparent front electrodes. After deposition, the samples were kept under an ultrahigh vacuum of 107 Torr during 24 h to reduce the initial oxygen content. Then, without exposure to air, the p-ZnPc layers were successively evaporated. Finally, the coplanar gold Au back contacts with a thickness of 50nm and an area of 2 10 mm were deposited on top of the device. Thus, we fabricated photovoltaic cells with a structure of ITO/n-C60 /p-ZnPc/Au. To provide a comparative study of organic layer thickness on cell performance, we have prepared three types of cells with the following parameters: A n-C60 80 nm /p-ZnPc 120 nm , B nC60 400 nm /p-ZnPc 400 nm , and C n-C60 800 nm /pZnPc 800 nm . With the same deposition conditions, single layers of n-C60 and p-ZnPc were prepared on ITO substrates for the UVvisible UV-vis optical spectra and photoconductivity measurements. Dark and photocurrent of single layers have been measured at constant dc electrical eld 105 V/cm in sandwich cell conguration between ITO front and Au back electrodes. Action spectra of short-circuit photocurrent for C60 /ZnPc cells were recorded under illumination from the ITO side. While measuring the photocurrent, we appropriately ltered and calibrated irradiation from a 500W xenon lamp through a monochromator to obtain a constant incident photon intensity PIN of 10 W/cm2 in the wavelengths of 360 850 nm. A Shimadzu UV-3101 PC spectrophotometer was used to record the optical absorption spectra. The current densityvoltage J-V characteristics were measured under illumination of a 15 mW/cm2 halogen lamp using an HP 1415B semiconductor parameter analyzer. Results The dark conductivity of the n-C60 layer D1 was measured just after its deposition to prevent the oxygen doping, while the p-ZnPc layer was exposed to air to induce the oxygen doping before we measure its dark conductivity D2 . Although the oxygen doping signicantly reduces the D1 value, it was reported that the doping process lasts longer than the time needed to take our measurements.21 In contrast, a short exposure to air initiates a
JOURNAL OF APPLIED PHYSICS 98, 054311 2006
Charge transport in hydrogenated boron-doped nanocrystalline silicon-silicon carbide alloysSeung Yeop Myong,a Oleg Shevaleevskiy, and Koeng Su LimDepartment of Electrical Engineering and Computer Science, Korea Advanced Institute of Science and Technology (KAIST), 373-1 Guseong-dong, Yuseong-gu, Daejeon 305-701, Republic of Korea
Shinsuke Miyajima and Makoto KonagaiDepartment of Physical Electronics, Tokyo Institute of Technology (TIT), 2-12-1 O-okayama, Meguro-ku, Tokyo 152-8552, Japan
Received 25 January 2005; accepted 28 July 2005; published online 9 September 2005 We have investigated the carrier transport mechanism of mixed-phased hydrogenated boron-doped nanocrystalline siliconsilicon carbide alloy p-nc-Si-SiC: H lms. From temperature-dependent dark conductivity measurements, we found that the p-nc-Si-SiC: H alloys have two different carrier transport mechanisms: one is the thermally activated hopping between neighboring crystallites near the room-temperature region and the other is the band tail hopping below 150 K. 2005 American Institute of Physics. DOI: 10.1063/1.2037871I. INTRODUCTION
Thin lms of hydrogenated amorphous silicon aSi: H and silicon carbide a-SiC: H have attracted considerable research interest mainly due to their potential applications in electronics, optical devices, and window layers in photovoltaic thin-lm solar cells. To achieve a highefciency thin-lm solar cell, a window layer should have a high electrical conductivity and a wide optical band gap. Due to the incorporation of carbon atoms, a-SiC: H has a wider band gap than a-Si: H. However, the incorporated carbon atoms limit the electrical conductivity of a-SiC: H. We can improve the electrical conductivity of a-SiC: H by an impurity doping. However, the impurity doping reduces the optical band gap of lms. In recent years, one promising way to incorporate a wide band gap and a high conductivity has been proposed by producing the mixedc or phase structure consisting of microcrystalline nanocrystalline nc- Si grains embedded in a-Si: H network. We rstly reported on the preparation of hydrogenated boron B -doped nc-SiSiC:H p-nc-Si SiC: H alloy lms containing nc-Si grains embedded in a-SiC: H matrix via the photodecomposition of C2H4.1 Its optical transmittance is mainly governed by the a-SiC: H matrix, while nc-Si grains are responsible for the effective transport of charge carriers.2 This p-nc-Si SiC: H alloy has a higher electrical conductivity, optical transmittivity, carrier mobility, and doping efciency than the conventional undiluted p-a-SiC: H. Based on the deposition of p-nc-Si SiC: H alloy,14 H2-diluted p-a-SiC: H buffer layers of p-i-n-type a-Si: H or protocrystalline silicon pc-Si:H solar cells were prepared.58 We found that the natural hydrogen treatmentetching the defective undiluted p-a-SiC: H window layer and improving order in the window layertakes place just before the highly conductive, low absorption, and well-ordered H2-diluted p-a-SiC: H buffer layer deposition onto the un